The invention relates to limited rotation motors such as galvanometers, and particularly relates to limited rotation torque motors used to drive optical elements such as mirrors for the purpose of guiding light beams in scanners.
Limited rotation torque motors generally include stepper motors and galvanometer motors. Certain stepper motors are well suited for applications requiring high speed and high duty cycle saw-tooth scanning at large scan angles. For example, U.S. Pat. No. 6,275,319 discloses an optical scanning device for raster scanning applications.
Limited rotation torque motors for certain applications, however, require the rotor to move between two positions with a precise and constant velocity rather than by stepping and settling in a saw-tooth fashion. Such applications require that the time needed to reach the constant velocity be as short as possible and that the amount of error in the achieved velocity be as small as possible.
Galvanometer motors generally provide a higher torque constant and typically include a rotor and drive circuitry for causing the rotor to oscillate about a central axis, as well as a position transducer, e.g., a tachometer or a position sensor, and a feedback circuit coupled to the transducer that permits the rotor to be driven by the drive circuitry responsive to an input signal and a feedback signal. For example, U.S. Pat. No. 5,424,632 discloses a conventional two-pole galvanometer motor that includes a sleeve formed of stainless steel or titanium.
FIG. 1 shows a conventional moving magnet galvanometer rotor that includes a magnet 10 surrounded by a brass tube 12, with stainless steel shaft stubs 14 and 16 at each end to couple the load and feed back respectively, and to provide journals for the supportive bearings. This rotor may be used, for example, in a galvanometer scanner that displays a first resonance at 7.5 kHz, and a small-step response time of 600 microseconds to a position error of 1% of command. The dense stainless steel and brass component parts of the rotor may be chosen to achieve simple solderability (if the parts are held together with lead-based solder), high stiffness, moderate damping, and low inertia.
A requirement of a limited rotation torque motor for certain applications (e.g., galvanometer scanner servo system) may be a system capable of changing the angular position of a load such as a mirror from an angle A to an angle B, wherein angles A and B are both within the range of angular motion of the galvanometer scanner, and are both defined arbitrarily precisely. The change in angular position should occur in an arbitrarily short time while maintaining a desired linearity of velocity within an arbitrarily small error. Both the minimum time of response of this system and the minimum velocity error are determined by the effective gain-bandwidth of the system. The gain-bandwidth of the system is the concatenation of the servo amplifier gain-bandwidth with that of the galvanometer scanner.
There are applications in which it is desirable to have greater torque than may be provided by conventional limited rotation torque motors. There is a need therefore, for limited rotation torque motors that provide improved flux density without adversely affecting the performance of the motor.